(NaturalNews) The health effects of mercury exposure have been further expounded by a new study recently published in the journal PLoS ONE. Researchers from the College of William and Mary (CWM) in Virginia found that songbirds exposed to sub-lethal and even trace amounts of mercury left over from industrial manufacturing suffered reproductive problems, brain abnormalities, and immune suppression, among other damage.

Birds' exposure to environmental chemicals has been previously studied in fish and fish-eating birds, but not so much in forest birds. So Dan Cristol, a biology professor at CWM, and a few colleagues set out to study how other types of birds, specifically songbirds living in and around the Shenandoah Valley's South River, are affected by chemical residues still present from manufacturing that took place at DuPont factories back in the early part of the century.

DuPont had been manufacturing Rayon during that time, a type of smooth textile fiber that is often used in place of silk, leaving behind mercury and other chemicals that ended up in the river and on nearby lands. And for many decades, birds living in the area have been exposed to these chemicals as they search for seeds and other food, a scenario that Cristol saw as an opportunity to learn more about the effects of this exposure.

Beginning in 2008, Cristol started publishing the results of monitoring he and his team had done on songbirds, including Carolina wrens and red-eyed vireos, which often turned out to have higher doses of mercury in their systems than fish-eating birds like kingfishers. The reason for this is that, like fish-eating birds, songbirds get a lot of their food from the river, including insects, arachnids, and other creatures.

"They had 20 percent fewer babies," stated Cristol about the health effects of this mercury exposure on songbirds. "Their songs are sung at the wrong pitch. Their hormone levels are altered. Their immune systems are suppressed."

More mercury exposure means more health damage, suggests study

Though a causal connection between mercury and these effects was not clearly delineated, tests conducted in the lab revealed that levels of mercury exposure correspond directly with disease rates. Particularly with regards to reproductive success, songbirds with higher levels of mercury in their bodies fared the worst, while songbirds with less mercury had superior fertility.

"Reproduction is the most widely known and is used by regulatory agencies to assess whether injury has occurred," added Cristol, as quoted by VAGazette.com.

Besides reproductive problems, the mercury-exposed songbirds also suffered brain abnormalities that left them unable to remember where to find food. Similar brain dysfunction has been observed in humans exposed to mercury, as the heavy metal is known to damage nerves and literally destroy brain cells, not to mention its ability to cross the blood-brain barrier and accumulate in brain tissue.

"Just for a frame of reference, the can of tuna you buy at the grocery store has about 0.5 parts per million" of mercury, stated doctoral researcher Claire Varian-Ramos, who helped conduct the study, about the lowest levels of mercury seen in songbirds that were associated with health effects. "It's similar to what you might find in bird prey items on many contaminated sites."

Interestingly, the surviving offspring of mercury-exposed songbirds did not suffer the same reproductive health effects as their parents, suggesting that mercury's toxicity reach may be constrained to a single generation. However, the fledgling rates of the mercury-exposed offspring was noticeably lower than that of control birds.

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